Ordered and disordered states in NiAs/Ni2In-type Ni1+δSn: Crystallography and order formation

An overview on the occupational order disorder behaviour of NiAs/Ni2In-type Ni1+delta Sn alloys is given. This order disorder behaviour leads to the occurrence of one disordered high-temperature phase HT and three different low-temperature phases LT, LT' and LT. The low-temperature phases, forming in different compositional regimes, are characterised by long-range order of Ni atoms against vacancies on trigonal-bipyramidal sites formed by Sn, leading to a commensurate superstructure in LT whereas LT' and LT are incommensurate. Different order disorder related phase transformation were monitored by X-ray powder diffraction. In particular during the formation of long-range order starting from disordered HT-Ni1+delta Sn, non-equilibrium crystal structures and microstructures evolve prior to formation of the ideally established equilibrium low-temperature phases. Quantitative analysis of the transformation kinetics including determination of activation energies was done by X-ray powder diffraction. The activation energies of the different processes can be related with the mobility of the Ni atoms in the crystal structure.